This invention relates to fasteners and more particularly to a corrosion resistant threaded fastener having a non-ferrous metallic head and a ferrous metallic shank.
In the fabrication of elements for outside use and particularly in the building construction field, it is important that the threaded fasteners securing the elements be resistant to the corrosive effects of the weather. Although there have been many fastener constructions proposed for elimination of this difficulty none are fully acceptable. For example, stainless steel, because of its resistance to corrosion has gained some acceptance. However, because of the high cost of stainless steel most fasteners available are presently constructed of carbon steel.
In order to reduce corrosion many of the carbon steel fasteners are coated or plated with zinc or some other noncorrosive material. However, since the threaded fasteners are self-tapping the coating on the threads is penetrated during the securing process. Thus, seals are provided about the head of the fastener to prevent rain water or other contaminates from leaking past the head and onto the threaded stud causing rust or other corrosive effects. Furthermore, during the securing process, and with time, the coating of the heads of the fasteners become scratched, gouged or the like and allow weather conditions to penetrate the carbon steel, eventually causing corrosive failures of the heads.
In attempts to overcome these deficiencies, the prior art has proposed the molding of a self-sealing plastic head onto the threaded stud member. Various known examples of these prior art proposals using plastic heads are illustrated in U.S. Pat. Nos. 3,134,290; 3,557,654; 3,618,444 and 4,154,138. It has now been found that the plastic, even if not initially cracked during the securing process, tends to fail after relatively short periods of time. Plastics have a resin or plasticizer which dry or bake out when the plastic is subjected to heat and sunlight. The dried-out plastic when undergoing expansion and contraction during changes in weather conditions subsequently cracks, resulting in direct contact of the corrosive environment on the carbon steel threads and failure thereof. Thus, plastic headed fasteners have fallen out of favor and are not now extensively used under these environmental conditions.